Vehicles that run using electricity as power equip rechargeable batteries such as lead batteries, nickel-metal hydride batteries, or lithium ion batteries. These rechargeable batteries supply electric power that is required by hybrid cars or electric cars when running. In order to control the operation of rechargeable batteries, it is necessary to calculate the state of charge (SOC) of the battery and to appropriately control charging/discharging electric currents on the basis of the calculated value.
Patent Literature 1 listed below describes a technique that calculates states of charge of batteries considering polarization voltages. Patent Literature 2 listed below describes, as a technique for reducing the risk of decrease in accuracy for calculating SOC, a method for keeping a difference between a prior SOC and a current SOC within a variation limiting value SOCL.